Support all params & add XUDT

[wmnsk]

This PR;

• adds all parameters with their own type
• adds String() funcs to all constants with the stringer tool and enrich String() of each param/message
• adds tests for everything
• adds a default logger which warns when something's wrong
• adds a server example which serves and prints received messages

and doesn't;

• modify message interfaces (which may be reorganized)
• flatten package structure (perhaps we don't really need param package)
  for now.